1. Field of the Invention
The present invention relates to a method and apparatus for electrophotographic image forming, a developing mechanism used in the apparatus, and a process cartridge including the developing mechanism. In particular, the present invention relates to a method and apparatus for electrophotographic image forming capable of effectively regulating a toner layer to form a uniformly thin layer and prevent toner overflow for producing a full-color image in good quality, a developing mechanism used in the apparatus, and a process cartridge including the developing mechanism and detachably provided to the image forming apparatus.
2. Discussion of the Background
Electrophotographic image forming apparatuses such as copiers, printers, and facsimile machines generally include an image bearing member, a charging unit, an optical writing unit, a developing unit, and a transfer unit for producing an image.
When forming an image, the charging unit uniformly charges a surface of the image bearing member, and the optical writing unit selectively emits a light beam according to image data and irradiates the surface of the image bearing member so that an electrostatic latent image is formed on the surface of the image bearing member. The developing unit then supplies one-component developer, for example, to visualize the electrostatic latent image to a toner image, and the transfer unit transfers the toner image formed on the surface of the image bearing member onto a transfer member provided in the image forming apparatus.
The developing unit using one-component developer generally includes a developer container, a developer carrying member, a developer supplying member, and a developer regulating member.
The developer container generally contains one-component developers. The developer carrying member is rotatably supported by the developer container and partially exposed from an opening of the developer container. The developer carrying member carries the developer on a surface thereof. The developer supplying member supplies the developer to the developer carrying member, and the developer regulating member regulates the developer on the surface of the developer carrying member.
When the developer supplying member supplies the developer to the surface of the developer carrying member, the developer regulating member regulates the developer on the surface of the developer carrying member to a layer having a predetermined height at a point in which the developer carrying member and the developer regulating member may contact, so that a developer layer having the predetermined height may be formed and be frictionally charged.
When a background image forming apparatus with such a developing unit allows a transfer medium having a size larger than an A3 size to be printed, an image bearing member of the background image forming apparatus may be extended in an axial direction according to the transfer medium, and, consequently, a developer carrying member and a developer regulating member may also be extended in an axial direction. These changes in lengths of the above-described members may cause a deflection to the developer regulating member that is applied with pressure exerted by pressuring members supporting the developer regulating member at both ends. The deflection may be greater toward the center of the developer regulating member in an axial direction. When deflection is generated on the developer regulating member, a contact pressure of the developer regulating member against the developer carrying member may vary along a surface of the developer regulating member. In particular, the axial center of the developer regulating member may have greater deflection so that a gap may be formed between the developer regulating member and the developer carrying member. Once the gap is formed, the developer regulating member cannot regulate the developer to the predetermined height on the surface of the developer carrying member and the layer of the developer may become higher at the axial center of the developer regulating member. The gap prevents the developer at the axial center of the developer carrying member from being evenly frictionally charged and increases the height of the developer layer. When the height of the developer layer increases, an amount of the developer also increases so that the developer at the center of the developer carrying member may have a substantially low charging quantity. Such substantially low charging quantity results in causing fogging in development and developer scattering.
To uniformly contact the developer regulating member with the developer carrying member, a technique in which a developer regulating member has nonuniform outer diameters of respective longitudinal cross-sectional regions of various potions thereof has been proposed. That is, a developer regulating member provided in a developing unit has an outer diameter of a longitudinal cross-sectional region at the center thereof greater than that at both ends thereof and is symmetrically formed thus centering the center thereof in an axial direction. The closer a portion is to the center, the more an amount of deflection increases and the greater an outer diameter of its longitudinal cross-sectional region of a developer regulating member becomes.
With the above-described structure, a developer regulating member may be pressed against a developer carrying member equally at any point on its surface contacting the developer regulating member, compared to a developer regulating member having a surface of a uniform outer diameter thereof. Thereby, a developer layer may be formed more uniformly.
When the above-described developer regulating member is used in the developing unit, however, a developer scraped by the developer regulating member may flow in an axial direction from the center of the developer regulating member toward each end thereof. The flow of developer is banked up by end sealing members that are provided for regulating a width of the developer layer formed on the developer carrying member. The developer stopped by the end sealing members may be accumulated to produce a pressure of developer powder in the vicinity of respective inner edges of the end sealing members and the amount of the developer powder pressure is substantially greater than that in any other areas of the developer carrying member. When the developer powder pressure increases in the vicinity of the respective inner edges of the end sealing members, a contact pressure of the developer regulating member against the developer carrying member cannot stop the developer at the end of the developer layer and allows the developer to flow in a rotation direction of the developer carrying member, which may allow excess developer for a developer layer to pass a contact point between the developer carrying member and the developer regulating member. Any developer that has passed the above-described contact point may be scattered from an opening of the developer container along with a rotation of the developer carrying member, which may result in a developer overflow. The developer overflow may contaminate an inside of the image forming apparatus and induce further contamination to the image bearing member and the developer carrying members having different colors from that of the leaked developer, which may result in deterioration of image quality.